Finishing machine and method for use in the hot dip metallic coating of steel strip, and coated strip



T. W. BAKER Sept. 29, 1959 FINISHING MACHINE AND METHOD FOR USE IN THE HOT DIP METALLIC COATING OF STEEL STRIP, AND COATED STRIP Filed March 12, 1953 INVENTOR. 41527? I RTTO?NEY$..

United States Patent FINISHING MACHINE AND METHOD FOR USE IN THE HOT DIP METALLIC COATING OF STEEL STRIP, AND COATED STRIP Thomas W. Baker, Sabraton, W. Va., assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Application March 12, 1953, Serial No. 341,936

9 Claims. (Cl. 29-528) This invention relates to a finishing machine and method for use in the hot dip metallic coating of steel strip and to coated strip produced thereby. More particularly, the invention has to do with the production of very thin metallic coatings on thin gauge steel strip. While the method and apparatus are adaptable for galvanizing or zinc coating as well as terne coating (i.e. coating with a lead-tin alloy), it is particularly useful and will be described principally in connection with the production of tin plate.

Tin plate is presently manufactured in one of two ways. Most commonly, it is manufactured on so-called tin plate sheet machines wherein sheet steel of appropriate gauge is passed through a tin pot and then through a finishing machine. Such machines produce excellent tin plate but their production or tonnage is very low since they run at a top speed of about 32 linear feet per minute.

It. is a well known phenomenon, in the coating of sheets or strip by the hot dip coating method, that the faster the sheet or strip is run through the machine, the thicker the coating Will be. Since tin plate coatings are very thin, the machine above described must be run very slowly for otherwise the coating will be too thick.

One type of tin plate is known as common coke and involves one and one-fourth pounds of tin per base box. A base box is the term used in the industry as a unit of surface and equals 3l,360square inches of surface. Thus, where two sides are involved, the area is. 62,720 square inches. For example, 112 sheets, 14 inches by 20inch'es' or'56 sheets, 20 inches by 28 inches, equal one base box.

Another. grade of tin plate known as charcoal" or special involves from two to five pounds of virgin tin per base box. It can, therefore, be seen that the thickness of a tin plate coating of the common coke grade is less than one-tenth the thickness of a conventional galvanized coat.

Tin plate is also produced by an electrolytic process but the electrolytic tin plate coating is so thin that it cannot properly be used in some applications without further protection. Thus, electrolytic tin plate has proved satisfactory for various dry pack applications but if it is to be used for wet pack applications, it is generally necessary to provide the tin plate with a coat of lacquer.

Thereare at present in use galvanizing machines which operate at very high strip speeds and which are used in the galvanizing of continuous strips of steel, but if itjis attempted to use such-machines for tin plate the coating willobviously be much too heavy. While it would be desirable to use such continuous machines and' while perhaps. they might' be used for tin plating, if they were operated at say 32' feet per minute, such operation is not feasible because of the low tonnage produced in relation to the tremendous investment in capital in the machine.

It is,.therefore, an object of the invention toprovide a finishing machine which may be used with presently known high speed continuous strip coating apparatusv so Patented Sept. 29, 1959 2 that common tin plate coatings may be applied to steel strip at very high speeds. 7

It is another object of the invention to provide a method of finishing hot dip metallic coating onsteel strip which may be carried out at such highspeeds as to render the use of continuous processes commercially profitable.

It is known that in a common coke tin plate wherein one and one-fourth pounds of Virgin tin are usedper base box, not all of the ha is in the form of a virgin tincoating. About .2 pound of the tin per base box is alloyed with the base metal in an iron-tin alloy (FeSn It is therefore a still further object of the invention to provide steel strip having such an iron-tin alloy surface with no virgin tin remaining on the strip.

These and-other objects of the invention which I shall disclose in more detail hereinafter or whichwill be apparent to one skilled in the art upon reading these specifications I accomplish by that series of'rnethod steps and by that certain construction and arrangement of partsof which I now disclose certain exemplary embodiments.

Reference is made to the drawings forming a part hereof and in which:

Figure 1 is a diagrammatic view of a tin pot and exit machine according to the invention.

Figure 2 is a similar diagrammatic view of a modification thereof. 7

Figure 3 is a view similar to Figure 2 showing further modification. v

Figure 4 is a fragmentary diagrammatic view of the uppermost pair .pinch rolls of Figure 2 showing another modification, and

Figure 5 is a view similar to Figure 3 showing a still further modification. V

Briefly, in the practice of my invention I cause the strip to issue from the tin pot between a pair of metering or skimming rolls which are partially submerged in the molten coating metal. These rolls perform the function of removingexcess coating metal from the strip as it issues from the pot and they also skim the surface of the molten metal to keep it free from scum at the point where the strip issues. Such metering or skimming rolls in and of themselves are well known.

Immediately above the skimming rolls, I provide one pair at least of pinch rolls which are made of' a material which is not wetted by the molten coating metal. Finally, above one or more pair of such non-wetting pinch rolls I may provide an additional pair of pinch rolls of a material which is wetted by the molten coating metal.

When I speak of a material which is wetted by the molten coating metal I mean a material which will alloy with the coating metal. conventionally, the exit rolls used in finishing machines in the hot dip coating of steel strip are of high carbon, low metalloid steel and have a Rockwell hardness of about 55 on the C scale.

The non-wetting or non-tinning rolls must be of a' material which will not alloy with the molten coating metal and for the purposes of tin plating, I prefer to use stainless steel or chromium-plated rolls;

My invention is based upon the discovery that nonwetting rolls are not subject to the speed limitations of wetted rolls, but will continue to squeegee excess tin from the strip surfaces at very high speeds, even up to 1000 ft./min. or more, while continuing to produce coatingsas light as 1% lb./base box or lighter.

InFigure l', the tin bath is indicated at 10 and'a conventional hood' through which the strip" is indicated at 11. The strip itself is indicated at 12; It will beunderstood that the strip has previously been prepared for the coating process, as for example by.

enters the pot phere until it is introduced into the bath through the hood 11. The strip passes in conventional manner over a bottom roll 13 and thence upwardly through the skimming rolls 14. The rolls 14, as is conventional, are partially submerged in the molten coating metal.

At 15 I have indicated pinch rolls of a non-wetting material between which the strip passes and the strip will then preferably travel vertically upwardly for such a distance as will insure that the coating has frozen before it changes direction by passing over a roll 16. While I have shown in Figure 1 two pairs of non-wetting pinch rolls, it will be understood that this is by way of example only. A single pair of such rolls may be used and forsome processes it may be desirable to use a greater number.

' The non-wetting rolls 15 act as squeegees and remove from the strip any excess of coating metal. Thus, if the rolls 15 are under sufficient pressure and are suflicient in number, I am able to squeegee off the strip substantially all of the virgin tin which has not alloyed with the base metal so that the surface will be constituted of the iron-tin alloy using about .2 pound of tin per base box.

Such material having an iron-tin alloy surface is useful in many applications. It can be soldered and can therefore be used without further processing in many dry pack applications. It can also be given a protective coating of lacquer and is then comparable to lacquered electrolytic tin plate.

In Figure 2 I have shown an arrangement similar to that of Figure l and corresponding parts have been given like reference numerals but I have added a pair of pinch rolls 17 above the series of pinch rolls 15 and the rolls 17 are of a material which is wetted by the coating metal. It will be understood that the number of pairs of pinch rolls 15 and their pressure against the strip will be such that they do not squeegee off all of the virgin tin, as described in connection with Figure 1, but leave a .very small amount of unalloyed tin on the surface of the strip. This small amount of tin on the surface of the strip is then smoothed down and uniformly distributed by the pair of wetted pinch rolls 17 so that the strip coming from the pinch rolls 17 will have a very thin coating of virgin tin over the iron-tin alloy surface. The rolls 17 will be provided with wipers or brushes 18 which may be of asbestos or of soapstone and which keep the rolls 17 free from an excess of molten tin.

The arrows in Figures 1 and 2 indicate the direction of rotation of the various rolls 14 through 17. Preferably these rolls will be driven at such a speed that their peripheral speed is equal to the lineal speed of the strip so that all of said rolls will have no-slip contact with the strip.

Conventionally, tin plate, after the coating operation, is subjected to temper rolling in order to give it the hardness or stiffness required for its various applications. A temper mill is indicated diagrammatically in Figure as having the working rolls 30 and backing rolls 31. For this operation, which is a dry operation, the coated strip must be dry and clean and particularly it must be free of oil. The conventional tin-plating process involves the use of oil in the finishing machine to control temperatures and therefore the strip must pass through a de-oiling process before the temper rolling operation. The reason for this is that if the strip is oily when it is temper rolled damage to the coating, by way of pinchers, buckles, and the like will result.

It will, therefore, be understood that if oil is elirninated from the finishing machine the de-oiling step can also be eliminated and this is accomplished by the apparatus and according to the procedures shown in Figures 1 and 2.

The apparatus disclosed herein can, however, be used with oil if desired as shown in Figures 3 and 4. The apparatus of Figure 3 is identical with that of Figure 2 except that the entire finishing machine is surrounded by a shell 19 dipping into the tin pot having an open top and containing a body of oil 20. With this arrangement it will be understood that the strip, after passing over the roll 16, goes to a cleaning or de-oiling apparatus.

It is not necessary that the entire finishing machine be submerged in oil as shown in Figure 4. Thus, the pair of wetting rolls 17 may be provided with oil pans 21 containing a body of oil 22 which is circulated by means of a pump. The wipers 23, corresponding to the wipers 13 of Figure 2, have been moved to a slightly diiferent position.

The apparatus disclosed herein also lends itself to use with protective atmospheres. In Figure 5 I have shown an arrangement similar to Figure 3 except that the member 19 has been replaced with a hood 24 which dips into the tin pot and is closed by a top 25 except for a slot 26 through which the strip issues. An inert gas is pumped in at the bottom through a pipe 27 and issues with the strip through the slot 26.

It will be understood that all the wetting rolls, i.e. the rolls 14- and 17, must be maintained above the melting point of tin, i.e. above 450 F. It will also be understood that the various rolls may be in the form of brushes of respectively non-wetting and wetting material.

It will also be clear that it is within the purview of my invention to provide a strip having a tin coating on one side, and an iron-tin alloy surface on the other side. This is accomplished by having one, only, of the roll pair or pairs 15 of non-wetting material, while the other roll of the pair or pairs 15 is of wettable material.

Numerous variations and modifications will suggest themselves to those skilled in the art and I therefore do not intend to limit myself otherwise than as set forth in the claims which follow.

Having now fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In the hot dip tin coating of steel strip at speeds on the order of 1,000 feet per minute, the steps of cansing the strip to issue from the molten coating metal between a pair of skimming rolls of a material which is wetted by the molten coating metal, and then to pass between a pair, at least, of pinch rolls of a material which is not wetted by the molten coating metal and located above the surface of said coating metal, and driving said rolls at a speed which will impart to the strip a speed of the order of 1,000 feet per minute and such that their peripheral speed is substantially equal to the linear speed of the strip, and in the direction of strip travel, whereby there is substantially no-slip contact between the roll peripheries and the strip.

2. In the method according to claim 1, the step of temper rolling said strip without intermediate treatment.

3. In the hot dip tin coating of steel strip at speeds on the order of 1,000 feet per minute, the steps of cansing the strip to issue from the molten coating metal between a pair of skimming rolls of a material which is wetted by the molten coating metal, and then to pass between a pair, at least, of pinch rolls of a material which is not wetted by the molten coating metal and located above the surface of said coating metal, and then causing the strip to pass between a further pair of pinch rolls of a material which is wetted by the coating metal, and driving all of said rolls at a speed which will impart to the strip a speed of the order of 1,000 feet per minute and such that their peripheral speed is substantially equal to the linear speed of the strip, and in the direction of strip travel, whereby there is substantially no-slip contact between the roll peripheries and the strip.

4. In the method according to claim 3 the step of controlling the temperature of the coating by means of oil applied to said further pair of pinch rolls, at least.

5. In the method according to claim 3, the step of maintaining an atmosphere of non-reactive gas about the strip durin said steps.

6. In the method according to claim 3, the additional step of wiping excess molten coating metal from said further pair of pinch rolls.

7. In the method according to claim 3, the step of temper rolling said strip without intermediate treatment.

8. In the hot dip tin coating of steel strip at speeds on the order of 1,000 feet per minute, the steps of causing the strip to issue from the molten coating metal between a pair of skimming rolls of a material which is wetted by the molten coating metal, and then to pass between a pair, at least, of pinch rolls, one roll, at least, of said last mentioned pair being of a material which is not wetted by the molten coating metal, said pair of rolls being disposed over the surface of said coating metal, and driving said rolls at a speed which will impart to the strip a speed of the order of 1,000 feet per minute and such that their peripheral speed is substantially equal to the linear speed of the strip, and in the direction of strip travel, whereby there is substantially no-slip contact between the roll peripheries and the strip.

9. In the hot dip tin coating of steep strip at speeds on the order of 1,000 feet per minute, the steps of causing the strip to issue from the molten coating metal between a pair of skimming rolls of a material which is wetted by the molten coating metal, and then to pass between a pair, at least, of pinch rolls, one roll, at least, of said last mentioned pair being of a material which is not wetted by the molten coating metal, said pair of rolls being disposed above the surface of said coating metal, and then causing the strip to pass between a further pair of pinch rolls of a material which is wetted by the coating metal, and driving all of said rolls at a speed which will impart to the strip a speed of the order of 1,000 feet per minute and such that their peripheral speed is substantially equal to the linear speed of the strip, and in the direction of strip travel, whereby there is substantially no-slip contact between the roll peripheries and the strip.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Nov. 30, 1938 

1. IN THE HOT DIP TIN COATING OF STEEL STRIP AT SPEEDS ON THE ORDER OF 1,000 FEET PER MINUTE, THE STEPS OF CAUSING THE STRIP TO ISSUE FROM THE MOLTEN COATING METAL BETWEEN A PAIR OF SKIMMING ROLLS OF A MATERIAL WHICH IS WETTED BY THE MOLTEN COATING METAL, AND THEN TO PASS BETWEEN A PAIR, AT LEAST, OF PINCH ROLLS OF A MATERIAL WHICH IS NOT WETTED BY THE MOLTEN COATING METAL AND LOCATED ABOVE THE SURFACE OF SAID COATING METAL, AND DRIVING SAID ROLLS AT A SPEED WHICH WILL IMPART TO THE STRIP A SPEED OF THE ORDER OF 1.000 FEET PER MINUTE AND SUCH THAT THEIR PERIPHERAL SPEED IS SUBSTANTIALLY EQUAL TO THE LINEAR SPEED OF THE STRIP, AND IN THE DIRECTION OF STRIP TRAVEL, WHEREBY THERE IS SUBSTANTIALLY NO-SLIP CONTACT BETWEEN THE ROLL PERIPHERIES AND THE STRIP. 